A lens for spectacles, regardless of whether or not it is a correcting lens, comes from a part possessing all of the optical qualities required for its use, and in particular an optical center referred to as the center of the lens. This part generally has a circular outline of a diameter that is large enough for all possible peripheral shapes corresponding to the immense variety of rims that exist in the spectacle frame market to be inscribed therein.
Trimming is a machining operation which consists in causing the outline of the lens to match the shape of the frame that is to receive it. This peripheral machining makes use of tooling in which the lens is clamped in the vicinity of its center between two accessories for holding it and generally for enabling the lens to be turned about an axis passing through the lens, while the desired outline is obtained using a grinding wheel, generally in two stages.
The lens clamping accessories are in the form of pads which, on being pressed against the concave and convex faces of the lens, give rise to stresses and deformations in the lens. The outline is therefore machined on a deformed lens under stress which, on being released, will take up a shape that is different, and will therefore have an outline that is different from the outline that was machined.
In order to process small lenses of oblong shape while ensuring that the lens is rotated correctly during trimming, it is necessary to use pads of oblong shape. It is not advisable, and is in any event ineffective, to apply strong pressure with a circularly symmetrical pad of small size. Unfortunately, the asymmetrical clamping that results from using oblong pads inevitably gives rise to deformation of the lens.